Power, Sex, Suicide: Mitochondria and the Meaning of Life (57 page)

All this probably sounds very sophisticated, but in fact it’s a nightmare of interpretation. Haemoglobin is present in massive amounts, whereas cytochrome oxidase is barely detectable. Worse still, the wavelengths of infra-red rays that the different haem compounds absorb overlap and merge with each other. It can be very hard to tell which is which. Even the machine gets confused. It measures a change in the redox state of cytochrome oxidase when what actually seems to be happening is a change in haemoglobin levels. We began to despair of ever gleaning any useful information from our contraption. Nor did the NADH levels help much. Most of the time there was a fine peak—a high concentration detected by the machine—before transplant, which vanished without trace after the organ had been transplanted, and that was that. It all sounded good on paper but the reality, as so often in research, was uninterpretable.

And then I had my own personal eureka moment, the moment I had my first inkling that mitochondria rule the world. It came about by chance, for one of the anaesthetics being used was sodium pentobarbitone. The concentration of this anaesthetic in the blood fluctuated, and on a few occasions when it did, we found we were picking it up on our machines. The levels of both oxyhaemoglobin and deoxy-haemoglobin remained unchanged, but we recorded a shift in the dynamics of the respiratory chain. Part of the NADH peak returned (it became more reduced) while the cytochrome oxidase became more oxidized. We seemed to be measuring a ‘real’ phenomenon, rather than the usual frustrating noise, because the levels of haemoglobin weren’t changing. What was going on?

It turned out that sodium pentobarbitone is an inhibitor of complex I of the respiratory chain. When its blood levels rose, it partially blocked the passage of electrons down the respiratory chains, and this led to a back-up of electrons in the chains. The early parts, including NADH, became more reduced, while the
later parts, including cytochrome oxidase, passed on their electrons to oxygen and became more oxidized. But why did this beautiful response not occur every time? This, we soon realized, depended on the quality of the organ. If the organ was fresh and functioning well, we picked up the fluctuations easily; but if it was seriously damaged it was virtually impossible to take a measurement. We saw the usual disappearance of all the peaks, never to return again. The explanation could only be that these mitochondria were as leaky as a colander—of the few electrons that entered the chain, barely any left it again at the end. Virtually all must have been dissipated as free radicals.

Without slicing out samples and subjecting them to detailed biochemical tests, we couldn’t be absolutely sure about what was really happening in these mitochondria, but we could say one thing for certain—the damaged organs were losing control of their mitochondria within minutes of transplantation, and there was absolutely nothing we could do about it. We tried all kinds of antioxidants, in an attempt to improve mitochondrial function, but to no avail. Mitochondrial function in those first few minutes foretold the outcome, perhaps weeks later—if the mitochondria failed in the first few minutes, the kidney inexorably failed; if they still had some life in them, the kidney had a good chance of surviving and functioning well. The mitochondria, I realized, were masters of life and death in kidneys, and extremely resistant to being tampered with.

Since then, in considering diverse fields of research, I’ve come to realize that the dynamics of the respiratory chain, which I struggled to measure all those years ago, is a critical evolutionary force that has shaped not just the survival of kidneys, but the whole trajectory of life. At its heart is a simple relationship, which may have begun with the origin of life itself—the reliance of virtually all cells on a peculiar kind of energetic charge, which Peter Mitchell named the chemiosmotic, or proton-motive, force. In each chapter of this book we’ve examined the consequences of the chemiosmotic force, but each chapter has concentrated on the larger implications of specific aspects. In the final few pages, I’ll try to tie all this together, to show how a handful of simple rules guided evolution in profound ways, from the origin of life, through the birth of complex cells and multicellular individuals, to sex, gender, ageing, and death.

The chemiosmotic force is a fundamental property of life, perhaps more ancient than DNA, RNA, and proteins. In the beginning, naturally chemiosmotic cells might have formed from microscopic bubbles of iron-sulphur minerals, that coalesced in the mixing zone of fluids seeping up from deep in the crust, and the oceans above. Such mineral cells share some properties with living cells, and their formation needs no more than the oxidizing power of the sun—there is no call for complicated evolutionary innovations before the origin of hereditary replication through DNA. Chemiosmotic cells conduct
electrons across their surface, and the current draws protons over the membrane to generate an electric charge across the membrane—a force field around the cell. This membrane charge links the spatial dimensions of the cell to the very fabric of life. All life, from the simplest bacteria to humans, still generates its energy by pumping protons across membranes, then harnessing the gradient to tasks such as motility, ATP production, heat production and the absorption of essential molecules. The few exceptions merely go to prove this general rule.

In cells today, electrons are conducted by the specialized proteins of the respiratory chains, which use the current to pump protons across the membrane. The electrons are derived from food, and pass down the respiratory chains to react with oxygen, or other molecules serving exactly the same purpose. All organisms need to control the flow of electrons down the respiratory chains. Too fast a flow fritters away energy wastefully, while too slow a flow can’t match demand. The respiratory chains behave like slightly cracked drainpipes—a clear flow presents no problems, but any blockage, either at the outflow or somewhere in the middle, is likely to spring a leak through the cracks. If blocked, the chains leak electrons, and these react to form free radicals. There are just a handful of possible reasons for electron flow to block, and only a few ways to restore the flow, yet the balance between power-generation, on one hand, and free-radical formation on the other—the same problem I faced in my kidneys—has written some of the most important, if unsung, rules in biology.

First among the reasons for a blockage of electron flow is some kind of defect in the physical integrity of the respiratory chains. The chains are assembled from a large number of protein subunits, which form into large functional complexes. In eukaryotic cells, genes in the nucleus encode most of the subunits, and genes in the mitochondria encode a small number. The continued existence of mitochondrial genes in all cells containing mitochondria is a paradox, for there are many good reasons to transfer them all to the nucleus, and no obvious physical reasons why this could not have been achieved, at least in some species. The most likely reason for their persistence is a selective advantage to retaining them in the mitochondria, and this advantage seems to be related to energy generation. So, for example, an insufficient number of complexes in the second part of the respiratory chains would block electron flow, leading to a backlog of electrons in the earlier part, and free-radical leakage. In principle, the mitochondria could detect the free-radical leakage, and correct the problem by signalling to the genes to make good the deficit—to produce more complexes for the second part of the chains.

The outcome depends on the location of the genes. If the genes are in the nucleus, the cell has no means of distinguishing between different mitochondria, some of which need new complexes, and many of which don’t: none are
satisfied by the bureaucratic one-size-fits-all response of the nucleus. The cell loses control over energy generation, a grave penalty. Only if a small contingent of genes is retained in each mitochondrion, to code for the core protein subunits of the respiratory chains, can energy generation be controlled in a large number of mitochondria simultaneously. The additional subunits, encoded in the nucleus, fit themselves around the core mitochondrial subunits, using them as a beacon and a scaffold for construction.

The consequences of this system are profound. Bacteria pump protons across their external cell membrane, and so their size is limited by geometrical constraints: energy production slopes off with a falling surface-area-to-volume ratio. In contrast, eukaryotes internalize energy generation in mitochondria, and this frees them from the constraints facing bacteria. The difference explains why bacteria remained morphologically simple cells, while the eukaryotes were able to grow to tens of thousands of times the size, accumulated thousands of times more DNA, and developed true multicellular complexity, surely the greatest watersheds in all of life. But why did bacteria never succeed in internalizing their own energy generation? Because only endosymbiosis—a mutual, stable collaboration between partners living one inside another—is able to leave the right contingent of genes in place; and endosymbiosis is not common in bacteria. The precise concatenation of circumstances that forged the eukaryotic cell seems to have happened just once in the entire history of life on earth.

Mitochondria inverted the world of bacteria. Once cells had the ability to control energy generation across a large area of internal membranes, they could grow as large as they liked, within limits set by the distribution networks. Not only could they grow larger, but they also had good reason to do so—energetic efficiency improves with larger size in cells and multicellular organisms, just as it does in human societies, following the economies of scale. There is immediate payback for larger size—lower net production costs. The tendency of eukaryotic cells to become larger and more complex can be explained by this simple fact. The link between size and complexity is an unexpected one. Large cells almost always have a large nucleus, which ensures balanced growth through the cell cycle. But large nuclei are packed with more DNA, which provides the raw material for more genes, and so greater complexity. Unlike bacteria, which were obliged to remain small, and to jettison superfluous genes at the first opportunity, the eukaryotes became battleships—large, complicated cells with lots of DNA and genes, and as much energy as needed (and no longer any need for a cell wall). These traits made a new way of life possible, predation, in which the prey is engulfed and digested internally, a step that the bacteria never took. Without mitochondria, nature would never have been red in tooth and claw.

If the complex eukaryotic cell could only be formed by endosymbiosis, the
consequences of two cells living together in mutual dependency were equally significant. Metabolic harmony may have been the rule, but there were important exceptions, and these too are attributable to the dynamics of the respiratory chain. The second reason for a block in electron flow is a lack of demand. If there is no consumption of ATP, then electron flow ceases. ATP is needed for replication of cells and DNA, and for protein and lipid synthesis—indeed, most housekeeping tasks. But demand is greatest when cells divide. Then the entire fabric of the cell must be duplicated. The dream of every living cell is to become two cells, and this applies as much to the erstwhile free-living mitochondria as to the host cells in the eukaryotic merger. If the host cell becomes genetically damaged, so that it can’t divide, then the mitochondria are trapped inside their crippled host, for they are no longer able to survive independently. And if the host cell can’t divide, it has little use for ATP. Electron flow slows down, and the chains become blocked and leak free radicals. This time the problem can’t be resolved by building new respiratory complexes, so the mitochondria electrocute their hosts from inside with a burst of free radicals.

This simple scenario lies at the roots of two major developments in life—sex, and the origin of multicellular individuals, in which all cells in the body share a common purpose and dance to the same tune.

Sex is an enigma. Various explanations have been put forward, but none explains the primal urge of eukaryotic cells to fuse together, as do the sperm and the egg, despite the costs and dangers of doing so. Bacteria don’t fuse together in this way, even though they do routinely recombine genes by lateral gene transfer, which apparently serves a similar purpose to sex. Bacteria and simple eukaryotes are often stimulated to recombine genes by various forms of physical stress, all of which involve free-radical formation. A burst of free radicals can be sufficient to induce a rudimentary form of sex, and in organisms like the green algae,
Volvox
, the free-radical signal for sex can come from the respiratory chains. In the early eukaryotic cells, mitochondria might have manipulated their hosts to fuse together and recombine their genes whenever the hosts were genetically damaged, and unable to divide by themselves. The host cell benefits, because the recombination of genes can fix or mask genetic damage, while the mitochondria themselves gain access to pastures new without killing their existing host, essential for their safe passage.

Sex may have benefited both the mitochondria and their hosts in single-celled organisms, but it no longer did in multicellular individuals. The gratuitous fusion of cells is a liability when the cells belong to an organized body, in which all constituent cells must share a common purpose. Now the same free-radical signal for sex betrays genetic damage to the host cell, which pays the penalty of death. This mechanism seems to be at the root of apoptosis, or programmed cell suicide, which is necessary for policing the integrity of
multicellular individuals. Without the death penalty for cellular insurrection, multicellular colonies could never have developed the unity of purpose characteristic of the true individual—they would have been torn asunder by the selfish wars of cancer. Today, apoptosis is controlled by the mitochondria, using the same signals and machinery that they had once used to plead for sex. Much of this machinery was originally brought to the eukaryotic merger by the mitochondria. While the regulation of apoptosis is now, of course, far more complicated, at its heart the critical signal is still a burst of free radicals from a blocked respiratory chain, leading to depolarization of the mitochondrial inner membrane, and the release of cytochrome c and other ‘death’ proteins into the cell. Even today, it takes no more: injecting damaged mitochondria into a healthy cell is enough for that cell to kill itself.